This invention relates to a spark plug having a center electrode, a ground electrode, and a noble metallic tip fixed to an electrode base material serving as at least one of the center electrode and the ground electrode. The spark plug of this invention is preferably applicable to an internal combustion engine installed in an automotive vehicle, a cogeneration system, and a pressurized gas feeding pump, or the like.
Generally, a spark plug used for an internal combustion engine has a center electrode, an insulator for holding this center electrode, a housing for fixedly holding this insulator, and a ground electrode having a proximal portion fixed to the housing and a distal portion opposing the center electrode. To meet the high performance of recent engines or to realize a maintenance free, assuring long life of spark plug is earnestly required nowadays. To this end, a noble metallic tip is fixed to each apical end (i.e., a spark discharge portion) of the center electrode and the ground electrode.
In this case, due to difference in the thermal expansion coefficient between the electrode base material and the noble metallic tip, a significant thermal stress acts on a joint area between the electrode base material and the noble metallic tip. Recent engines are subjected to severe exhaust gas purification and employ a lean burn combustion technique. Electrodes of a spark plug are exposed to high-temperature combustion. Rapidly increasing and decreasing the plug temperature will cause a severe thermal load acting on the joint area of the electrode base material and the noble metallic tip.
The thermal stress acting in an outer peripheral region of a tip is large. The larger the thermal stress, the faster the oxidation advances from the outer periphery toward the center of the tip. In other words, the margin of joint (or bond) reliability becomes so small that the noble metallic tip may fall or peel off the electrode base material. To relax the thermal stress, Japanese patent No. 59-47436 discloses a relaxing layer capable of bringing the diffusion effect in the thermal treatment.
However, according to the above-described conventional manufacturing method, the cost will increase due to addition of a thermal treatment. In view of this problem, it may be desirable to select the materials having similar thermal expansion coefficients for the electrode base material and the noble metallic tip. However, this method includes the following problems.
For example, if a noble metallic tip is made of a material having a thermal expansion coefficient similar to that of an electrode base material, it will be necessary to add a large amount of additives, such as Ni, to a noble metal. This will worsen the anti-exhaustion properties of a noble metallic tip and therefore cannot assure a satisfactory life of a spark plug.
On the contrary, if an electrode base material is made of a material having a thermal expansion coefficient similar to that of a noble metallic tip, the electrode base material will need to contain an element having a small thermal expansion coefficient, such as W or Mo. This will worsen the bendability (i.e., workability) of an electrode base material. Such a material cannot be used for a spark plug.